In an conventional image reading device, a reading device positioned at the reading position reads an image of an original document while the original document is conveyed. In such type of image reading device, if dust such as paper dust exists between the reading device and the original document, the reading device reads the dust. Accordingly, unwanted line is generated in the image read by the image reading device,
Japanese Patent Application Publication No. 2006-60493 discloses an image reading device, in which dust level is detected at a plurality of reading positions by displacing a reading device to the plurality of reading positions in a sub-scanning direction prior to start of image reading operation. A position where variation in dust level is small will be determined as an ultimate reading position.